A photonic bandgap medium comprises an artificially engineered periodic dielectric array having at least one photonic bandgap, i.e., a range of frequencies in which ordinary electromagnetic wave propagation is strictly forbidden. The presence of these photonic bandgaps can be used to confine and guide electromagnetic waves for any of a variety of useful purposes. Guiding and confinement is achieved by the judicious introduction of defect regions, i.e., missing or differently-shaped portions of the periodic array, within which the electromagnetic waves are permitted to exist and wherealong the electromagnetic waves can be confined and guided.
Photonic bandgap media can also be engineered to provide negative refraction for incident electromagnetic radiation propagating therethrough. Such negatively refracting photonic bandgap media can be realized even though all of their component materials are intrinsically of positive refractive index. Unlike with homogeneous positively refracting material, the electromagnetic field within a negatively refracting photonic bandgap medium is highly modulated and cannot be described by a simple ray diagram. However, from a system-level viewpoint, optical performance can be at least partially characterized by assigning the negatively refracting photonic bandgap medium with a negative index of refraction at interfaces with surrounding media.
Due to the dispersive nature of photonic bandgap media and the manner in which negative refraction can be associated therewith, a broader sense of the term photonic bandgap is used herein. In particular, whereas some may refer to a photonic bandgap as a frequency range of width W between 3 db stop-band points around a center frequency, the term photonic bandgap herein refers to a broader frequency range therearound, e.g., a frequency range of width 2 W.
Among other uses, a negatively refracting photonic bandgap medium can be used to form a flat lens, a flat slab of material that behaves like a lens and focuses electromagnetic waves to produce a real image. Flat lenses can be potentially advantageous in that, unlike curved homogenous-material optical lenses, there is no diffraction limitation that limits focusing of the light onto an area no smaller than a square wavelength. Instead, light can be focused onto a very tight area even smaller than a square wavelength. Another potential advantage is that, unlike with curved homogenous-material optical lenses, there is no central axis and therefore many alignment difficulties are obviated. Accordingly, so-called “photonic bandgap superlenses” are great candidates for very small-scale optical circuits. It is to be appreciated, however, that the scope of the present teachings is not limited to flat-lensing applications, and a wide variety of other uses and devices can be achieved according to the present teachings.
One issue that arises in the realization of useful devices from negatively refracting photonic bandgap media relates to substantial losses experienced by the incident electromagnetic signal when propagating therethrough. Accordingly, it would be desirable to reduce signal loss in negatively refracting photonic bandgap media. It would be further desirable to provide a general approach to reducing such losses that can be applied to a variety of negatively refracting photonic bandgap media operating across a variety of different spectral ranges.